Jim Woessner walked home to Sausalito from the highway 1 at highway 101 near Mill Valley. King tides in the Bay Area caused some flooding in Marin County Wednesday December 12, 2012 and many beaches were partially covered.

Photo: Brant Ward, The Chronicle

Jim Woessner walked home to Sausalito from the highway 1 at highway...

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A record high tide known as a "king tide" floods roads near the Manzanita Park And Ride on Wednesday Dec. 12, 2012 in Mill Valley, Calif.

Photo: Mike Kepka, The Chronicle

A record high tide known as a "king tide" floods roads near the...

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A pickup truck braves the water logged road after a record high tide known as a "king tide" floods roads near the Manzanita Park And Ride on Wednesday Dec. 12, 2012 in Mill Valley, Calif.

Photo: Mike Kepka, The Chronicle

A pickup truck braves the water logged road after a record high...

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Parts of an old tidal gauge sit next to the oldest continuously operating gauge in the Americas, near Crissy Field in S.F.

On the night of Jan. 31, 1953, a hurricane descended on the northeast coast of the Netherlands, whipping up high spring tides into 18-foot surges.

As dike after dike failed, floods washed over at least 600 square miles, leaving hundreds of thousands homeless and killing more than 1,800.

The tragedy set off a decades-long campaign to ensure that it never happened again, as the nation adopted an essentially zero-tolerance policy for mass flooding. They erected a closed system of levees built to withstand all but a 1 in 1,000 years flood, sometimes forsaking ecosystems in the name of safety.

As the Bay Area begins planning for rising sea levels that could expose hundreds of square miles to flooding by 2050, the question becomes: What level of risk tolerance are we comfortable with?

California has done more than most states to prepare for the challenges of climate change, but when it comes to the dangers of sea level rise, experts worry that the state isn't doing nearly enough.

'A lot of risk'

In late 2008, Gov. Arnold Schwarzenegger signed an executive order directing the California Resources Agency and other groups to study the risk of sea level rise, while requiring state agencies to consider those projections in future project planning.

But accounting for sea level rise in new developments does nothing to address the vastly larger problem of vulnerable shorelines and communities that already exist. And at this point, no state department has clear authority to oversee such efforts, much less a budget.

"There's a lot of risk among existing infrastructure and communities and there is no single agency responsible for that," said Heather Cooley, co-director of the water program at the Pacific Institute.

Absent a state department or law, it will be difficult to impossible to get the dozens of local governments that touch San Francisco Bay to act in any kind of logical lockstep on what will be a huge problem, several observers said.

Among other efforts, the commission is working with several dozen local, state and federal organizations to carefully evaluate the vulnerability of the Alameda County shoreline from Emeryville to Union City. They hope to establish a process that other areas around the bay can follow.

To date, the working group has created detailed maps that highlight areas exposed to high tides and 100-year floods, under 16-inch and 55-inch sea rise scenarios. Even the low-end estimates show water inundating considerable portions of Oakland International Airport, Hayward and Union City.

The wide-open question is what to do about it.

Lindy Lowe, BCDC's project lead on the Adapting to Rising Tides effort, stressed that they're just beginning the process of identifying potential strategies.

But rather than one dramatic fix, she said, it will probably come down to a combination of hard and soft approaches that can be used at specific sites as rising tides require: new levees, raised seawalls, expanded wetlands and improved drainage.

At the same time, Lowe said, part of adapting to the region's new reality may simply mean better preparing communities and emergency responders for occasional flooding. Some cities will simply balk at the high price of rigorous protection.

"We as a society thus far have taken the 'Well, it will happen every so often and we'll try to make the best of it' approach," she said. "So people get wet, get injured or die. That attitude may change as this affects us more often."

Attitudes certainly shifted after the North Sea flood of 1953, as well as the more recent examples of Hurricane Katrina and Superstorm Sandy. But the Bay Area has the option of learning from those earlier tragedies, instead of waiting around for its own.

Adequate standard?

One critical assumption those events call into question is the 100-year flood standard.

The long tradition in the United States is building shoreline protections to hold back flood levels that have a 1 percent chance of occurring in any given year. It's also the worst-case scenario explored in BCDC's shoreline analysis.

But a growing number of experts say it's already an inappropriate standard for urban centers - and one that could drastically understate the risk in the near future. The Netherlands, where about a quarter of the country is below sea level, has been shifting to a 10,000-year standard.

"The 100-year standard has driven levels of protection below economically optimal levels, has encouraged settlement in areas behind levees, and resulted in losses of life and vast federal expenditures following major flood and hurricane disasters," concluded a committee organized by the National Research Council to draw lessons from Hurricane Katrina.

Scrambled picture

The bigger problem is that no one can say what a 100-year flood looks like anymore. Scientists say that global warming is increasing the frequency and intensity of extreme weather events, scrambling the math of risk analysis based on historic patterns.

In other words, planning assumptions that experts already argue are inadequate for population centers might also be plain wrong. If so, the little planning work under way for San Francisco Bay could understate the risks to residents and property in the decades ahead.

"For a place like San Francisco, or the low-lying parts of the Bay Area, the annual probability of major flooding ought to be pretty low because you have a lot of people there," said John Christian, a member of the National Academy of Engineering who worked on the Katrina report. "We're dealing with a very rapidly changing environment that has made prediction difficult."